The present application relates to thermoformable polycarbonate/polyester compositions and to the use of such compositions in molded articles. The compositions of the invention are significant because the having a large processing window, thus facilitating the making of molded articles.
Numerous disclosures exist of products that combine polycarbonate and polyester components. As is common in combinations of polymers, these compositions make use of selected materials in selected amounts that emphasize certain desired characteristics for a particular product application, while minimizing undesirable side-effects. Thus, many compositions represent a compromise trading the good properties for a limited number of less desirable properties. In the compositions of the invention, the desired application is thermoforming or blow-molding, and the desired characteristics are a large processing window (sometimes called a forming window) and a quality product.
The processing window is the difference (in degrees C.) between the highest and the lowest temperatures over which thermoforming can be successfully performed. It is not an indication of the actual temperature required, but rather of the flexibility of the processing requirements. By way of example, XENOY™ X5300WX, a blend of polycarbonate and polybutylene terphthalate (PBT) sold by General Electric Co. has a narrow thermoforming processing window of less than 5° C.
Narrow processing windows arise because the melt strength of the material is low, such that cohesion is lost during forming if the temperature is significantly above the temperature at which the material is soft enough to form at all. To overcome this problem, the art has employed melt strength enhancers. One such material is styrene-acrylonitrile encapsulated polytetrafluoroethylene (TSAN, CAS NO: 9002-84-0) which is commonly employed to reduce drips (a consequence of added melt strength) and therefore enhance flame retardance in polycarbonate compositions. However, as demonstrated below, PC/PBT compositions made with TSAN as a melt enhancer can suffer from visible surface defects, making this approach less desirable.
WO 2005/066277 discloses polycarbonate/polyester compositions in which polycarbonate are combined with polycyclohexyldimethylterephthalate (PCT) or polycyclohexyldimethylterephthalate glycol (PCT-G). The results presented showed that melt strength was superior for compositions containing just branched polycarbonate was better than for one containing a mixture of branched and linear polycarbonate. The results further showed that higher levels of PCT-G had lower melt strength. Blends of PCT or PCT-G with polycarbonate and a poly-organosiloxane core shell impact modifier are disclosed in PCT Application WO98/17725. WO02/36685 discloses blends of 5 to 45% by weight polycarbonate and 55 to 95% of a copolyester such as PCT or PCT-G. WO 2004/069931 discloses compositions that contain polycarbonate, PCT and PCT-G for use in extrusion blow-molding.